In a process of refining crude oil in an atmospheric distillation unit, low-grade heavy oils such as Bunker-C oil are produced in large amounts. When such heavy oils are introduced and treated in a fluidized catalytic cracking (FCC) unit, high-value-added light oil products such as gasoline and propylene are produced together with heavy oil and off-gas. The off-gas generated in the FCC unit is composed of hydrogen, methane, nitrogen, carbon monoxide, carbon dioxide, ethane, ethylene, propane, propylene, C4+, water and trace amounts of impurities (acidic gas, COS, H2S, arsenic, ammonia, nitrile, mercury, etc.). Although the FCC off-gas contains a large amount of ethylene as shown in Table 1 below, the concentration of ethylene in the FCC off-gas is low and the FCC off-gas contains various gases. For this reason, ethylene has not yet been recovered from the FCC off-gas in an economic manner, and the FCC off-gas has been used as fuel gas.
TABLE 1HydrogenMethaneEthanePropaneEthylenePropyleneComponents(H2)(CH4)(C2H6)(C3H8)(C2H4)(C3H6)Concentration30.3 25.611.9 0.511.91.55(vol %)Concentration6.7022.619.7 1.2218.43.60(wt %)ComponentsC4+CarbonCarbonNitrogenImpuritiesWatermonoxidedioxide(N2)(H2S, ammonia,(CO)(CO2)etc.)Concentration1.55 1.2 0.215.3Traceaturated(vol %)amountsConcentration1.66 1.86 0.4923.7(wt %)
<Composition of FCC Off-Gas (Concentration: Dry Base)>
Conventional techniques for separating light olefins (ethylene, propylene, butylene, etc.) and paraffins (ethane, propane, butane, etc.) by adsorption are as follows.
U.S. Pat. No. 6,867,166 discloses a technique of separating olefins by a pressure swing adsorption or temperature swing adsorption process using a transition metal ion-supported adsorbent having selectivity for ethylene or propylene.
Also, U.S. Pat. No. 6,293,999 discloses a technique of separating propylene from a propane/propylene mixed gas by a pressure swing or temperature swing process using an ALPO-14 adsorbent having a molecular sieve function which selectively adsorbs only propylene. Moreover, U.S. Pat. No. 6,488,741 discloses a technique of separating C2-C4 olefins by a pressure swing adsorption process or a combination of a pressure swing adsorption process with a distillation process using a zeolite adsorbent. In addition, U.S. Pat. No. 6,488,741 discloses a technique of separating propylene from a propane/propylene mixed gas using an 8-member ring adsorbent having a molecular sieve function, such as SAPO.
As described above, the processes for separating ethylene or propylene by adsorption are carried out by gas-phase adsorption rather than liquid-phase adsorption, because ethylene or propylene is not easy to liquefy. Also, the regeneration of adsorbents is performed by pressure swing adsorption or temperature swing adsorption.
Elevating and lowering the temperature of an adsorption column in the temperature swing adsorption process requires a lot of time, so that the productivity of the bulk gas separation process is low, and thus the equipment cost is high. The pressure swing adsorption or vacuum swing adsorption process is not suitable for separating large amounts of mixed gases, because the capacity of a compressor or a vacuum pump is limited.
Korean Patent Registration No. 0849987 registered in the name of the applicant discloses an adsorption separation process capable of separating ethylene from an FCC off-gas containing a low concentration of ethylene using an ethylene-selective adsorbent. The disclosed process is a displacement desorption process of desorbing adsorbed ethylene using a desorbent and is a technique of concentrating and recovering ethylene from the FCC off-gas through the sequential steps of adsorption, ethylene rinse and displacement desorption.
The concentration of ethylene in FCC off-gas is as low as about 10-20 wt %, even though it varies depending on the operating conditions of processes. In order to produce high-concentration ethylene from such FCC off-gas containing a low concentration of ethylene, rinse with a large amount of high-purity ethylene should be carried out, so that the consumption of energy in a distillation process for separating an ethylene/desorbent stream into components is disadvantageously increased, thus increasing the energy consumption of the overall process.
If the purity of ethylene is low, a paraffin/desorbent stream containing large amounts of weakly adsorbing components (hydrogen, nitrogen, methane, etc.) is discharged, and a large amount of energy is consumed to recover the desorbent in the state in which the concentration of the weakly adsorbing components is high.
The present applicants have conducted extensive studies into a technique for recovering ethylene from FCC off-gas with high purity and low cost in order to solve the above-described problems and have found that, when ethylene in the FCC off-gas is partially concentrated by a pressure swing adsorption process and high-purity ethylene is produced from a mixed gas containing the partially concentrated ethylene, ethylene can be economically recovered with high purity, thereby completing the present invention.